Hitherto, a polyurethane in which a reinforcing nylon net is incorporated has been used as an air bag cover, and upon expansion of the air bag which is folded and stored in the air bag cover, the air bag cover bursts at the thin-wall part thereof where the reinforcing net is not present. However, such an air bag cover suffers from the problem that when the reinforcing nylon net is not used, cracks occur at the parts other than the thin-wall burst part or the cover is fragmented and scattered at the time of bursting. When the reinforcing nylon net is used, the problem in bursting at the time of expansion of the air bag is solved, but there are production problems in that the molding of the air bag cover takes time for setting the position of the reinforcing net, the rejection rate increases due to deviation of the position of the reinforcing nylon net, and productivity decreases because of the need for relatively slow polyurethane RIM molding.
Furthermore, various air bag covers formed of thermoplastic resins having no reinforcing nylon net have been studied. However, in the case of these air bag covers, it has been very difficult for them to have a softness of 60-99 in spring hardness (JIS K 6301) as that passengers do not feel uncomfortable when these resins used as interior trims for automobiles, to work properly for air bag expansion at a temperature of -40.degree. C. to 90.degree. C., and to prevent occurrence of cracks in the part other than the burst part and consequent scattering of fragments at the time of expansion of the air bag.
Among them, some of polyolefin thermoplastic resins can provide good expansibility at -40.degree. C. to 90.degree. C. just after molding, but cannot provide sufficient expansibility for the air bag and sufficient fragment scattering resistance of the cover at -40.degree. C. to 90.degree. C. after having been subjected to a high-temperature aging test or weathering test because of the change in properties caused by change in phase structure such as change in crystallinity of the polyolefin thermoplastic resins. That is, at present, no air bag covers have been developed which have a spring hardness of 60-99 and can provide excellent expansibility for the air bag not only just after molded, but also after having been subjected to high-temperature aging test or weathering test.
Furthermore, most of these air bag covers investigated at present are coated for prevention of scratches with nails or clothings, uncomfortable feeling such as tackiness, and discoloration caused by sunlight, and, therefore, air bag covers which need not to be coated have been desired from productivity and economical viewpoints. Hitherto, in order to improve scratch resistance or reduce the tackiness, it has been attempted to increase the surface slippage of the molded products by adding organic lubricants such as higher fatty acids, higher fatty acid esters and higher fatty acid amides or silicone oils of low viscosity. However, these additives migrate very much to the surface over time or because of heat, and damage the appearance of the molded products or deteriorate the scratch resistance.